In the field of color tunable multi LED luminaries or multi LED lamps, color control is a crucial topic in order to achieve and maintain color point accuracy. Color points or, more generally, colors are quantified by chromaticity coordinates, of which the most widely used are the CIE (Commission International de l'Eclairage) 1931 chromaticity coordinates. Here the combination of x and y defines the color and L defines the brightness, i.e. luminosity, of the light. This system is based on the response of the eye of the average observer and is the internationally accepted standard. The requirement to control the color point is basically triggered by various inherent problems related to the LEDs used in such a luminary. For example, the optical characteristics of individual LEDs vary with temperature, forward current, and aging. In addition, the characteristics of the individual LEDs vary significantly from batch-to-batch for the same LED fabrication process and from manufacturer to manufacturer. Therefore the quality of the light produced by the LED luminary can vary significantly and the desired color and the demanded brightness of the light cannot be obtained without suitable feedback systems.
Such a feedback system is typically realized by at least one sensor. Obviously, on the one hand the selection of the type of sensor strongly depends on the demanded accuracy or performance of the sensor and on the other hand on the economic impact of the sensor's price on the total product price. In this context, one of the big challenges is to correctly represent the effect of wavelength drift in the light emitted by the luminary. This challenge finds its basis in the fact that sensitivity of the human eye shows significant peaks in the so-termed color matching function that describes the color perception of the human eye, wherein said peaks are not present in the spectral response of a simple photodiode when used as a light sensitive element of the sensor. Hence, such a sensor might enable precise measurements of the radiometric flux of a certain primary color and allow keeping the radiometric flux of a lamp constant, but still the actual color of the emitted light as perceived by the human observer may deviate from the desired color because the said simple photodiode doesn't track the wavelength drifts of the primary color(s) induced by temperature variations or aging. In order to deal with this phenomenon, complex models representing aging or temperature behaviour are required. But also ambient light that is not perfectly shielded from the sensor's photodiode might disturb the sensor's measurements and consequently leads to a mismatch between the actual color perception and the desired color perception. The situation turns even worse if the sensor is used to achieve a constant illumination in a certain location while ambient light or contributions from other light sources influence the integral measurements obtained by the simple photodiode.
On the other hand, the problems identified in terms of a purely photodiode-based sensor might be overcome by using more advanced sensor arrangements. For example, the use of a spectrometer that provides a very high spectral resolution would allow thorough analyses of the spectral property of the light emitted by the LED luminary. However, the pricing target for the LED luminary does not allow the use of such spectrometers.
The latter problem might be overcome by applying a true color sensor, which is realized to take the color perception of the human eye into account. Such a true color sensor typically comprises at least three photodiodes, each of which is equipped with a color filter. For example, a circuit for sensing a property of light in the form of a multi-photodiode true color sensor is disclosed in U.S. Pat. No. 6,630,801 B2. The sensor realizes a first circuit element and comprises filtered photodiodes and unfiltered photodiodes. A further circuit element coupled to the sensor measures the output signal of the filtered and unfiltered diodes and correlates these readings to chromaticity coordinates for each of the red, green, and blue LEDs of the luminary. Based on this correlation, forward currents driving the LEDs of the luminary are adjusted in accordance with differences between the chromaticity coordinates of each of the red, green, and blue LEDs and chromaticity coordinates of a desired mixed color light. Although this solution achieves desired results in terms of color control, its realization still requires a significant number of sensor elements in combination with appropriately realized and manufactured filters, which in total does not allow a cost efficient and compact design.
Therefore, it is an object of the invention to provide a circuit for sensing a property of light having an improved and more cost efficient circuit design. It would also be desirable to provide a method of sensing a property of light that shows an improved performance while at the same time a more cost efficient implementation is enabled.